You have a large cavity and your dentist is trying to estimate whether you have enough tooth left to put a crown on. What does he mean by that? If you don't have enough tooth does it mean that you can't get a crown? What will happen to the tooth then?
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How much tooth structure is needed for a crown?
Is there a requirement for how much tooth structure you need or can you put a crown on any tooth regardless of it's condition? There is of course a minimum requirement because if you don't have any tooth structure left, it would be impossible to put a crown on. Crowns need enough tooth so that it can stay on.
The situation is similar to the headless horseman who has no head so he is unable to wear a hat. You need a head to wear a helmet! The same principle applies to teeth. If you don't have enough tooth left, you can't wear a crown!
Basically, the more tooth you have left the better the crown will fit. The dental crown will be more retentive, the more natural tooth structure you have left. As you start losing tooth structure, the retentiveness of the crown will start decreasing. What this means is that the less tooth structure you have, the more likely the crown will fall off while you're eating.
Prognosis of the crown depends on the number of walls remaining
Dentists usually measure the prognosis based on how many walls of the tooth are left. If you have four walls of tooth remaining, the prognosis is terrific. The less walls you have, the long term survival rate starts to drop.
The bare minimum of tooth structure that we need for a crown would be at least one wall. How do you put a crown on a tooth with one wall you ask? We can push teeth bonding technology to the limits with a core build up.
This tooth just finished a root canal and it has about a little bit less than 2 walls left. The prognosis isn't as good as we would like but we can still save it and restore it with a dental crown. The first thing we need to do is bond a core build up to restore the missing tooth structure.
The core build up is a very similar material to a cavity filling except it has one additional characteristic to it, which is that it self cures. Composite fillings require a blue LED light to make it hard but core build ups will self cure and self harden. It does not require a light but only time for it to harden and set.
When is it not enough tooth left for a crown?
As amazing as current teeth bonding technology is, there are still limitations. There are two scenarios where we are unable to put a dental crown on.
There is no more tooth structure left above the gum line.
The tooth is completely fractured.
The first situation is when the tooth completely breaks off. It looks like you don't have any tooth left because everything above the gum line is now gone.
The second scenario where we cannot put a dental crown on since the tooth is completely broken. It use to be one tooth but has now become two separate teeth since it split in half. Even if we put a crown on, this tooth won't last very long since we can't defy the laws of physics. We can't undo a fracture of this size and extent.
What can I do if I can't put a crown on?
If your tooth happens to be cracked in half or cracked below the gum line, it cannot be saved with a crown anymore but you do have alternative options. Your next best option would be to have the tooth removed and then get a dental implant placed.
An implant is a titanium screw that will replace the root of the tooth. Once it fully integrates with your bone, you actually get a crown on top of it! Except this time it is called an implant crown.
Closing statement
So essentially there is a minimum required tooth structure for you to get a dental crown and that is at least one wall of tooth left. Although if you happen to need the tooth extracted, you can still get a dental implant with an implant crown on it afterwards! Thank goodness for an advance in dental technology.
